[PATCH backup] readme: Add instructions on how to build the docs

[Maximiliano Sandoval <m.sandoval@proxmox.com>]

If the binaries have not been built then make -C docs throws an error
which is not obvious.

Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>
---
 README.rst | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/README.rst b/README.rst
index 3059ba387..9f3d359cb 100644
--- a/README.rst
+++ b/README.rst
@@ -65,6 +65,23 @@ You are now able to build using the Makefile or cargo itself, e.g.::
   # # or for a non-package build
   # cargo build --all --release
 
+Building the online documentation
+=================================
+
+The online documentation can be build in HTML format as follows. First build the
+required binaries::
+
+  make docs
+
+The previous step is only necessary once. Then the online documentation can be
+built or regenerated with::
+
+  make -C docs DEB_HOST_RUST_TYPE=x86_64-unknown-linux-gnu html
+
+With the `rust target <https://doc.rust-lang.org/rustc/platform-support.html>`__
+of the host. The resulting documentation will be at ``docs/output/html/`` and
+can be open with your browser of choice or, e.g. ``xdg-open``.
+
 Design Notes
 ************
 
-- 
2.47.3